1. Field of the Invention
This invention relates to the recording of data on optical discs, and more specifically toward obtaining superior laser tracking and improved pit geometry in optical recordings created within the art of optical disc mastering.
2. Description of the Prior Art
Various methods have been employed in recording data on optical discs, wherein, data is recorded by having a laser burn pits into an optical disc. Optical disc (OD) Masters are articles from which prerecorded mass produced consumer OD's are subsequently molded. The master is a first article in the OD manufacturing process having data or information recorded on a disc or platter that can read by optical means. All subsequent OD manufacturing and duplication processes transfer that identical form, format and data from the original first article by various means including: vacuum deposition; electroforming/plating; or injection molding processes.
Past techniques for producing masters combined existing technologies from various industries. The most prominent of these techniques is the photo resist concept of the semiconductor industry. The master had formed on it a series of pits which serve to optically record information. The pits form a spiral line or track starting at the center of the disc and continuing to the outer periphery of the disc. Conventionally, the pits are approximately 0.6 .mu.m (10.sup.-6 meters) wide, 0.9 to 3.3 .mu.m long, 0.12 .mu.m deep, and are separated from neighboring tracks, on either side thereof, by 1.6 .mu.m center to center.
To record using the photoresist technique, a lamina of photoresist material is spun upon a very smooth, polished circular plate of glass. The glass plate with photoresist lamina is then placed in an oven to cure the photoresist. The cured, photoresist coated, glass plate is next set upon a turntable which is caused to spin. A data modulated laser optical system is focused on the glass plate, while suspended by a relatively large and complex translational mechanism, to cause the focused spot laser to move very gradually, at a steady radial rate, outwardly from the center of the spinning glass plate while concomitantly radiating the surface of the plate. The affect being to produce a spiral track of data in the photoresist material. The turntable spins at either a constant angular velocity or a constant linear velocity (CLV) while any point on the surface of the plate is being radiated by a laser. This process is referred to as mastering, wherein, data is recorded by creating pits in the optical disc from the exposure to laser light along the spiral path as the plate rotates. After the desired data has been recorded in the photoresist lamina, the plate is placed in at least one bath of developing solution, typically a process that requires multiple steps, whereby, the exposed areas in the photoresist are etched out leaving a series of pits. The glass plate with etched photoresist is then oven dried, followed by metalization of the photoresist lamina by vacuum deposition of a thin conductive lamina of metal thereon, which yields the desired master. The master is finally subjected to an analysis and validation process prior to electroforming to make a stamper and replication.
The foregoing type of mastering system is quite expensive in terms of capital equipment involved, labor, space, sensitivity to motion and shock, time and costs. Operating costs to produce a single master can be relatively expensive.
As an alternative to the photoresist mastering system another technique, known as a non-photo resist (NPR), or direct offset method, utilizes what is referred to as a dry process formula (DPF). This process incorporates the use of a material which undergoes ablation when exposed to a laser, thereby, forming the desired pit has brought forth a number of advantages over the photoresist process. These advantages include a reduction in the process steps by climinating, among others, developing and curing steps, which results in a less costly procedure and much shorter completion time, and the ability to monitor the quality of the disc concurrent with the mastering process of recording data thereon. The plate and DPF material are set upon a turntable for mastering and concomitant analysis. Metalization of the pitted layer is conducted to complete the mastering process. Once such a master is made, conventional electroplating and replication processes are accomplished to make a stamper which is then used for mass duplication of OD's by conventional injection molding processes. The NPR system makes some improvement over the prior art largely due to eliminating etching processing and oven-curing but it does not eliminate motion sensitivity nor the need for expensive and complex translator mechanisms and the glass reconditioning processes. Capital expense for such systems is still substantial, and the cost to produce a single master is still relatively expensive.
Yet another mastering process is referred to as a direct metal mastering (DMM) process. In the DMM process, instead of using a laser to either expose or cause ablation of the surface material, a tiny diamond stylus actually engages the surface, usually metal, and gouges the pits. Although technically feasible, this concept has had no meaningful application in industry because the "no contact" laser systems offer inherent increased reliability, longevity and efficiency over mechanical systems.
A major objective in recording optical disc masters is to control the geometry of the pits used to record data on the disc. Poor pit geometry makes the separation of duplicate discs from the disc stampers difficult. Also, data transmission can be affected by pits having raised areas surrounding the pit called berms, which are a common result in the dry process formula. An ideal pit is one formed without a surrounding raised berm area. Much modern research and development has been devoted to creating recorded optical discs having such ideal pits. However, the conventional techniques, discussed above, have not developed a manner of recording optical discs without generating the undesirable berm areas surrounding the pits used to record data.
As can be see from the forgoing discussion, the prior art possesses inherent limitations, and there continues to be a long standing need for a more efficient, less time consuming, and less expensive method for manufacture of an OD Master, that is effective in controlling pit geometry. The invention disclosed herein does just that.